1. Field of the Invention
The present invention relates to an apparatus and method for controlling the flow of compressed air, and in particular for controlling the flow rate of compressed air to a paint spray system and to other which are optimally operatice dependent upon receiving a specified flow rate of air.
2. Background of the Invention
Many factories use compressed air as a source of power for operating various types of production equipment. “Compressed air, ” which is sometimes referred to as “pressurized air” or referred in spray paint operations as “atomization air, ” is defined as free air that has been pressed into a volume that is smaller than it normally occupies. Controlled expansion of the compressed air can be used as a source of power to operate a wide range of pneumatically powered tools.
Compressed air is typically supplied from onsite or nearby compressors and piped through a distribution system to a downstream point of use. Paint spraying operations for painting various types of manufactured products, including especially automobiles and airplanes, is one typical use of compressed air.
In a spray paint operation, a paint fluid, which can be in the form of either a liquid or a fine powder, is mixed together with compressed air in a spray gun nozzle in order to atomize the paint into extremely fine particles and to transfer the paint particles onto the surface of the item being painted. One commonly used spray paint gun, referred to in the industry as a high volume low pressure (HVLP) spray gun, generates high volumes of low pressure air which transfers the paint particles to the surface of the article being painted with a relatively low velocity. Other uses of compressed air include pneumatically powered machine tools, drills and wrenches, and other pneumatically powered items which are optimally operative dependent upon receiving a specified flow rate of the pressurized air into the tool.
Major changes in downstream air demand create varying loads on the compressors. Air compressors are typically controlled according to system supply pressure, coming online as the system air pressure at the compressor drops below a threshold pressure and going offline at a higher cutout pressure. This is a rather crude method of controlling air supply, especially as air compressors are normally quite slow to respond to change.
Industrial compressed air systems are commonly controlled by pressure regulation, meaning, by regulating the nominal air pressure at a certain point in the system. A pressure regulator might be placed, for example, at or near the compressor, at one or more points on the distribution line, or on a hose which used to supply air to the tool. The major disadvantage with this method is that measuring air pressure at only one particular point in the system is not necessarily a good indicator of the air pressure at another point in the system. Air pressure drops as it flows through the system, and the amount that the air pressure drops from one point to the next varies greatly depending on the specific installation and also on varying conditions of usage during the course of the day. In many cases a compressed air system supplies not only spray guns but also other devices used in a paint shop such as sanders, polishers, screw drivers, drills and so forth. The intermittent operation of such other tools will affect the air pressure throughout the system.
For paint spray operations in particular, one commonly used method for determining whether a sufficient amount of compressed air is being delivered to the spray gun is to place a pressure gage on the cap of the spray gun immediately after the painter has set the spray gun for proper atomization of the paint but before he actually begins painting. Many operators, however, find this extra step to be a great inconvenience which interrupts their painting operations, and so they often do not do it. Another method to checking whether a sufficient amount of compressed air is being delivered to the spray gun is to attach an air pressure regulator and gage to the handle of the gun. However, attaching a pressure regulator to the gun naturally increases its weight. Over a period of time, muscle fatigue sets in, thereby causing the operator to use unnatural arm and wrist actions, which cause overspray or underspray conditions and other flaws in the paint job.
More importantly, regulating the nominal pressure at any one point in the system does not necessarily mean that the proper amount of air, or even any air is flowing at another point downstream. There may be a blockage in the spray nozzle of a paint gun, or a break in the line or some other problem in the system.
The difficulty in delivering a proper amount of compressed air to a spray paint nozzle is further exasperated by the fact that paint viscosity varies due to temperature fluctuations. If the temperature of the paint varies, the amount of paint fluid delivered to the nozzle of the spray gun also varies. Therefore, to compensate for the change in viscosity of the paint fluid, the amount of compressed air delivered to the spray gun nozzle must be adjusted. This type of adjustment is not easily done with only pressure regulation, and at a minimum requires a great deal of testing and trial and error to achieve the proper settings. Additionally, in many typical spray paint operations a single compressor is used to deliver compressed air to a number of output points. Each point is located a different distance from the air compressor, and so the pressure drops from the compressor to one point or the other will differ. Further, hoses which deliver air from the output point on the wall to the spray gun in the paint booth often differ in length, diameter or both, which greatly affects the pressure drop from one end of the hose to the other. Additionally, different types of spray paint guns, which require different amount of pressurized air, may be employed. The number of variables which are encountered during the course of operations thus increase to the point where it is nearly impossible to control the amount of compressed air delivered to the spray paint gun based merely on regulating the nominal air pressure at any particular point in the system.
Accordingly, an improved means for controlling the flow rate of compressed air delivered to a spray paint gun, as well as to other types of pneumatic tools which are optimally operative dependent upon receiving a specified flow rate of compressed air is desired.